Acoustic flowmeter

ABSTRACT

An acoustic flowmeter for measuring the flow of fluid in a pipe having a pair of transducers mounted on the pipe opposite each other with one downstream from the other, an oscillator associated with each transducer, the transducers sharing time so that each is used in the transmit mode and the receive mode to use the same acoustic path and thereby eliminate the variables due to the use of different paths, means for controlling the amplitude of the excitation applied to the transmit mode transducer and means for controlling the frequency of the oscillators, and means for obtaining the difference between the two oscillator frequencies which frequency is a function of the velocity of flow of the fluid in the pipe.

I Umted States Patent 1 1 3,72 ,IWS Cirulis L451March 13, 1973 [5' 1ACOUSTIC FLOWMETER 3,329,017 7/1967 Yamamoto ct al. ..73/194 A [75]Inventor. Uldis Cirulls, Midland Park, NJ. Primary Examiner charlcs ARuahl [73] Assignee: NU Sonics, 1nc., Paramus, NJ. Attorney-Samuelson &Jacob [22] Filed. March 24, 1971 ABSTRACT [21] Appl' 127551 An acousticflowmeter for measuring the flow of fluid in a pipe having a pair oftransducers mounted on the [52] U.S. Cl. ..73/l94 A pipe opposite eachother with one downstream from [51] Int. Cl. ..G0lf l/00 the other, anoscillator associated with each trans- [58] Field of Search ..73/I94 Aducer, the transducers sharing time so that each is used in the transmitmode and the receive mode to use [5 6] References Cited the sameacoustic path and thereby eliminate the variables due to the use ofdifferent paths, means for con- UNITED STATES PATENTS trolling theamplitude of the excitation applied to the 2,921,467 1/1960 Hedrichetal. ..73 194 A transmit mode transducer and means for controlling2,949,773 3/1960 Batchelder 3 94 A the frequency of the oscillators, andmeans for obtain- 3,007,339 11/1961 Hill 73/194 A ing the differencebetween the two oscillator frequen 3,420,102 l/1969 Brown..... 73/194 Acies which frequency is a function of the velocity of 2,912,856 11/1959Kritz 73/194 A flow of the fluid in the pipe, 3,605,504 9/1971 Kummer,Jr. et al.. ..73/67.7 3,237,453 3/1966 Yamamoto et al. ..73l194 A 6Claims, 4 Drawing Figures DIFFERENCE AMPLIFIER DRIVER AMPLIFIER DRIVERAMPLIFIER L THRESHOLD VOLTAGE CONTROLLED 0 ,OSCILLATOR DETECTOR PAQ QESWITCH 269 z E 2 DIFFERENCE 8 AMPLIFIER c A A as f l c W R TOR a seaCOUNTER SWITCH .525 BE VOLTAGE CONTROLLED 546 OSCILLATOR 56 Pmminmmm3.720.105

SHEET 10F 3 INVENTOR. ULDlS Cuauus f HT I6RNEVS PATENTEDMAR] 3197a sum 3or 3 INVENTOR. Uums Clreuus n-rmeuevs ACOUSTIC FLOWMETER The inventionrelates to acoustic flowmeters used for measuring the velocity of afluid flowing in a pipe.

In particular, the invention is directed toward providing an acoustic(sonic) flowmeter utilizing two transducers which are used to transmitand receive the sonic pulses traversing the fluid on a shared basis. Inthis configuration, the upstream and the downstream transmis' sionsshare the same path and have the same sonic path length. It is thuspossible to eliminate the errors in measurement which are a function ofdifferences in sonic path length. Accuracy of the measurement of thefluid flow velocity, which is a function of the difference in theupstream and the downstream pulse repetition frequencies, can thereby beimproved by this construction.

Prior art techniques and constructions have attempted various solutionsto the path length problem.

It is an important object of the invention to provide I an acoustic(sonic) flowmeter which utilizes a pair of oppositely mountedtransducers, one downstream from the other, which selectively transmitsonic pulses across the fluid, to obtain a determination of the fluidflow velocity from a measurement of the difference in the downstream andupstream pulse repetition frequencies.

It is a further object of the invention to provide such a flowmeterwherein there is an electronic amplitude servo system for limiting theamplitude of the excitation applied to the transducer in the transmitmode. The amplitude of the excitation applied to the transducer in thetransmit mode is adjusted so that the amplitude of the signal receivedby the transducer in the receive mode is constant for all conditions ofsound attenuation in the fluid medium.

It is a still further object of the invention to provide such aflowmeter wherein there is an electronic frequency servo system forcontrolling the frequency of the excitation oscillators.

It is a still further object of the invention to provide such aflowmeter wherein critical elements of the system are common to both thedownstream and upstream transmissions to thereby eliminate errors due toinherent differences in dual elements, no matter how well matched theymay be.

These and other objects, advantages, features and uses will be apparentduring the course of the following description when taken in conjunctionwith the accompanying drawings, wherein:

- FIG. 1 is a plan view of a transducer pipe section which is a part ofthe flowmeter of the invention;

FIG. 2 is a sectional view taken on the lines 2-2 of FIG. 1, viewed inthe direction of the arrows;

FIG. 3 is a block diagram of the acoustic (sonic) flowmeter of theinvention; and

FIG. 4 is a timing diagram of the system of FIG. 3.

In the drawings, wherein, for the purpose of illustration, and whereinlike numerals are employed to designate like parts throughout the same,the numeral 10 designates a pipe section which is inserted in a pipesystem whose fluid flow is to be measured. Pipe section 10 is affixed inthe flow by means, for example, of bolts and nuts inserted in openings14 of flanges 12 which cooperate with similar flanges on the adjacentpipe sections.

For the purpose of illustration, it is assumed that the fluid flow is inthe direction of arrow 16 so that transducer 20A is the downstreamtransducer which transmits upstream and transducer 208 is the upstreamtransducer which transmits downstream.

Numeral 22 is used to designate the acoustic (sonic) path between thefaces of the two transducers and its length is designated as L. Theacute angle made by path 22 with the wall of the pipe is designated asa.

The flowmeter of the invention comprises a pair of velocimeters whichare locked by time comparison techniques as will appear later in thisdescription. The transducers share time on the acoustic path so that forpart of the time transducer 20A is in the transmit mode and transducer208 is in the receive mode and for another part of the time transducer20B is in the transmit mode and transducer 20A is in the receive mode.

The relationship between the fluid flow velocity and the pulserepetition frequency is given as follows:

Upstream: f N (c v'cosa)/L Downstream: f, N (c vcosa)/L where:

c velocity of sound in the fluid v velocity of fluid flow L sonic pathlength a angle between the sonic path length and the pipe wall Nfrequency divider ratio of the phase-locked loop Now, Af=f f (2NVcosa)/L and the fluid flow velocity,

V AfL/ZN cosa Thus, it can be seen that the fluid flow velocity is afunction of the frequency difference and can be determined from ameasurement of Af.

The designation AB is used to signify the enabling of the circuitsassociated with transducer 20A to put it in the transmit mode and thedesignation BE is used in the same sense with respect to transducer 208.

Considering the circuit of FIG. 3, and assuming that the AE circuits areon so that transducer 20A has excitation applied to it from voltagecontrolled oscillator 24A through switch 26A, control logic counter 28to input 2 of driver amplifier 30A and decoupling means preferably diode32A.

Switch 26A is preferably a logic gate which is on when AB is on and isoff where there is no AE signal present.

The excitation signal is changed by transducer 20A to an acoustic signalwhich is transmitted across the pipe along path 22 and is received bytransducer 20B. Transducer 20B converts the acoustic signal into anelectrical signal which is blocked by diode 328 so that the signal goesthrough switch 34A, which is preferably afield effect transistor whichis on when AB is on and is of when there is no AE signal present. The

signal then is applied to the input of radio frequency amplifier 36. Theoutput of radio frequency amplifier 36 is connected to peak detectordiode 38 which is, in turn, connected to switch 40A. Switch 40A issimilar to switch 34A.

The output of switch 40A is connected to capacitor 42A and to input 1 ofdifference amplifier 44A. The charge on capacitor 42A is proportional tothe amplitude of the received pulse received by transducer 203 andserves to vary and control the amplitude of the excitation pulse. Thisis accomplished by comparing the voltage on input 1 of differenceamplifier 44A with the reference voltage applied to input 2 from source46.

The output of difference amplifier 44A is applied to input 1 of driveramplifier 30A and serves to raise or lower the output excitation signal,as required. Input 3 of driver amplifier 30A is used to obtainadjustable delay.

Input 3 is a manual adjustment to equalize the delays in the A and Bchannels. The delay adjustment consists of adjusting a small trimmercapacitor. The delay introduced is a function of the size of thistrimmer capacitor.

The loop just described can be designated an electronic amplitude servocircuit since it serves to control the amplitude of the excitationsignal applied to the transducer. Thus, it can be seen that theamplitude may be changed to accommodate changes in the acousticattenuation in the path which is encountered, for example, with a changein the fluid flowing in the pipe.

The output of radio frequency amplifier 36 is also connected to theinput of a threshold detector 48 which passes signals above a certainpredetermined value to input 1 of comparator 50. A signal from output 3of control logic counter 28 is applied to input 2 of comparator 50. Thesignal output of comparator 50 flows through switch 52A, which issimilar to switches 34A and 40A, to input 1 of difference amplifier 56Aand to capacitor 54A. The charge on capacitor 54A is proportional to thepulse repetition frequency.

The signal on input 1 of difference amplifier 56A is compared with thatfrom reference voltage source 58 which is applied to input 2. The outputof difference amplifier 56A is connected to voltage controlledoscillator 24A and increases the frequency if the output voltage ispositive and decreases it if it is negative. This is then an electronicfrequency servo as it serves to control the excitation pulse repetitionfrequency.

Control logic counter 28 turns off circuit AE and turns on circuit BE sothat transducer 208 now transmits downstream to transducer 20A. Theoperation just described for condition AE now takes place for conditionBE utilizing all the elements designated with the suffix B and thoseelements which are common to both systems A and B. The criticalcomponents are common to both modes so that there is a minimum of errordue to the mismatching of these components which would probably occur iftwo such elements were used (one in each system). The common componentsare radio frequency amplifier 36, peak detector diode 38, thresholddetector 48, control logic counter 28 and comparator 50.

The outputs of voltage controlled oscillators 24A and 243, whosefrequency outputs are proportional to voltage, are fed to frequencydifference circuit 60 which produces frequency fwhich is a parameterfrom which the flow velocity may be derived.

The output of circuit 60 may be displayed on a display 62 such as acounter indicating flow velocity or a totalizing counter may be used toproduce a mass flow display.

The operation of the flowmeter of the invention will best be understoodby an examination of FIG. 4. Curve represents the pulse train which istransmitted by voltage controlled oscillator 24A. Pulses 72 are thepulses which are applied to transducer 20A to produce an acoustic pulsewhich is transmitted upstream across the fluid flow.

Curve 74 represents the AB (A enable) on" condition when positive andthe of condition when zero. Curve 76 is the representation of thecomparator reference signal. Curve 78 represents the pulse train whichis transmitted by voltage controlled oscillator 24B and pulses 80 arethose which are applied to transducer 208 to produce an acoustic pulsewhich is transmitted downstream across the fluid flow.

Curve 82 represents the BE (B enable) on condition when positive and theof condition when zero. Curve 84 represents the combined pulse trainsfrom oscillators 24A and 24B, those from oscillator 24A are designated1",, and appear when AB is on and those from oscillator 24B aredesignated f and appear when BE is Pulses 86, 88 and 90 represent thethree possible positions at which an acoustic pulse transmitted bytransducer 20A may appear at the output of the threshold detector 48.Pulses 92, 94 and 96 represent the three possible positions at which anacoustic pulse transmitted by transducer 20B may appear at the output ofthe threshold detector 48.

Pulses 88 and 94 have arrived at the correct time, namely at thepositive-going edge of the comparator reference curve 76. In theseconditions, no frequency corrections are made. Pulses 86 and 92 arrivetoo early. Under these conditions, the associated oscillator frequencyis increased. Pulses 90 and 96 arrive too late and under theseconditions, the associated oscillator frequency is decreased.

Operation proceeds as follows:

1. AB on 2. Pulse 72 is applied to transducer 20A and an acoustic pulseis transmitted across the fluid.

3a. Pulse 88 received no frequency correction. or

3b. Pulse 86 received frequency of oscillator 24A is increased, or

30. Pulse 90 received frequency of oscillator 24A is decreased.

4. At some subsequent time AB is turned off by control logic counter 28.

5. BE on 6. Pulse 80 is applied to transducer 208 and an acoustic pulseis transmitted across the fluid.

7a. Pulse 94 received no frequency correction. or

7b. Pulse 92 received frequency of oscillator 24B is increased. or

70. Pulse 96 received frequency of oscillator 24B is decreased.

8. The signal represented by curve 84 containing f, during AB on and fduring Be on is used as input to the control logic counter 28. Thesignals represented by curves 70 and 78 (f, and f respectively) areapplied to a frequency difference circuit whose output is utilized todisplay flow velocity or mass flow depending upon the display deviceused.

While a particular embodiment of the invention has been shown anddescribed, it is apparent to those skilled in the art that modificationsare possible without departing from the spirit of the invention or thescope of the subjoined claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a system for measuring the flow of fluid in a pipe wherein a pairof transducers is mounted on the pipe opposite each other and whereinone of the transducers is downstream from the other and wherein there isa pair of oscillators for exciting the transducers, there being one suchoscillator associated with each transducer, the improvement comprising:

means for selecting one of the oscillators to thereby excite one of thetransducers such that only one of them is excited at any given time inthe transmit mode such that an acoustic wave is transmitted across thepipe and is received by the other transducer in the receive mode andsuch that each of the transducers is alternatively in the transmit modeand the receive mode;

amplitude servo means connected to the transducers such that its inputis connected to the receive mode transducer and its output is connectedto the transmit mode transducer such that the amplitude of the signalreceived by the receive mode transducer is maintained constant;

frequency servo means having its input connected to the receive modetransducer and its output connected to the oscillator associated withthe transmit mode transducer such that the frequency of the saidoscillator is maintained within predetermined limits; and

means for obtaining the difference between the frequencies of the twooscillators, which difference frequency is a function of the flowvelocity of the fluid in the pipe. y

2. The invention of claim 1 wherein theramplitude servo means comprises?a single radio frequency amplifier, having an input and an output, whoseinput is connected to either i of the transducers whichever is in thereceive mode;

a pair of difference amplifiers, each having at least two inputs and anoutput there being one such difference amplifier associated with eachtransducer in the transmit mode;

a pair of driver amplifiers, there being one such driver amplifierassociated with each transducer in the transmit mode, each such driveramplifier having at least two inputs and an output;

the output of the radio frequency amplifier being selectively connectedto one of the inputs of the difference amplifier associated with thetransducer in the transmit mode;

a reference voltage directly connected to the other input of bothdifference amplifiers;

the output of each difference amplifier being connected to one of theinputs of its associated driver amplifier;

the other input of each driver amplifier receiving excitationfrom itsassociated oscillator when its associated transducer is in the transmitmode.

3. The invention of claim 2 including a peak detector diode connectedbetween the output of the radio frequency amplifier and the inputs tothe difference amplifiers.

4. The invention of claim 3 wherein the frequency servo means comprises:

a control logic counter having an input and a plurality of outputs;

the input of the control logic counter being selectively connected tothe output of the oscillator in the transmit mode;

a threshold detector having an input and an output, its input beingconnected to the output of the radio frequency amplifier;

a comparator having at least two inputs and an output;

an output of the control logic counter being connected to an input ofone of the driver amplifiers such that signal is transmitted to thedriver amplifier when its associated elements are in the transmit mode;

an output of the control logic counter being connected to an input ofthe comparator and the output of the threshold detector being connectedto a second input of the comparator such that the signals therefrom arecompared to produce a difference therebetween at the output of thecomparator;

a second pair of difference amplifiers, there being one such amplifierassociated with each transducer, each of said amplifiers having twoinputs and an output;

the output of the comparator being selectively connected to one of theinputs of one of the pair of the second pair of difference amplifierswhose associated transducer is in the transmit mode;

a reference voltage connected to the other input of each of the secondpair of difference amplifiers so that the difference amplifier developsa difference signal between the signal from the comparator and thereference voltage;

the output of the said difference amplifier being connected to itsassociated oscillator so that the frequency is adjusted in apredetermined direction depending upon the sign and amplitude of thedifference signal.

5. The invention of claim 2 wherein the frequency STVO means comprises:

a control logic counter having an input and a plurality of outputs;

the input of the control logic counter being selectively connected tothe output of the oscillator in the transmit mode;

a threshold detector having an input and an output, its input beingconnected to the output of the radio frequency amplifier;

a comparator having at least two inputs and an output;

an output of the control logic counter being connected to an input ofone of the driver amplifiers such that signal is transmitted to thedriver amplifier when its associated elements are in the transmit mode;

an output of the control logic counter being connected to an input ofthe comparator and the output of the threshold detector being connectedto a second input of the comparator such that the signals therefrom arecompared to produce a difference therebetween at the output of thecomparator;

a second pair of difference amplifiers, there being one such amplifierassociated with each transducer, each of said amplifiers having twoinputs and an output;

the output of the comparator being selectively connected to one of theinputs of one of the pair of the second pair of difference amplifierwhose associated transducer is in the transmit mode;

a reference voltage connected to the other input of each of the secondpair of difference amplifiers so that the difference amplifier developsa difference signal between the signal from the comparator and thereference voltage;

the output of the said difference amplifier being connected to itsassociated oscillator so that the frequency is adjusted in apredetermined direction depending upon the sign and amplitude of thedifference signal.

6. The invention of claim 1 wherein the frequency servo means comprises:

a threshold detector having an input and an output with its inputreceiving a signal from the receive mode transducer;

a control logic counter having an input and a plurality of outputs;

the input of the control logic counter being selectively connected tothe output of the oscillator in the transmit mode;

a comparator having at least two inputs and an output;

an output of the control logic counter being connected such that signalis transmitted to the transducer which is in the transmit mode;

an output of the control logic counter being connected to an input ofthe comparator and the output of the threshold detector being connectedto a second input of the comparator such that the signals therefrom arecompared to produce a difference therebetween at the output of thecomparator;

a pair of difference amplifiers, there being one such amplifierassociated with each transducer, each of said amplifiers having twoinputs and an output;

the output of the comparator being selectively connected to one of theinputs of the difference amplifier whose associated transducer is in thetransmit mode;

a reference voltage connected to the other input of each of thedifference amplifiers so that the difference amplifier develops adifference signal between the signal from the comparator and thereference voltage; the output of the difference amplifier beingconnected to its associated oscillator so that the frequency is adjustedin a predetermined direction depending upon the sign and amplitude ofthe difference signal.

1. In a system for measuring the flow of fluid in a pipe wherein a pairof transducers is mounted on the pipe opposite each other and whereinone of the transducers is downstream from the other and wherein there isa pair of oscillators for exciting the transducers, there being one suchoscillator associated with each transducer, the improvement comprising:means for selecting one of the oscillators to thereby excite one of thetransducers such that only one of them is excited at any given time inthe transmit mode such that an acoustic wave is transmitted across thepipe and is received by the other transducer in the receive mode andsuch that each of the transducers is alternatively in the transmit modeand the receive mode; amplitude servo means connected to the transducerssuch that its input is connected to the receIve mode transducer and itsoutput is connected to the transmit mode transducer such that theamplitude of the signal received by the receive mode transducer ismaintained constant; frequency servo means having its input connected tothe receive mode transducer and its output connected to the oscillatorassociated with the transmit mode transducer such that the frequency ofthe said oscillator is maintained within predetermined limits; and meansfor obtaining the difference between the frequencies of the twooscillators, which difference frequency is a function of the flowvelocity of the fluid in the pipe.
 1. In a system for measuring the flowof fluid in a pipe wherein a pair of transducers is mounted on the pipeopposite each other and wherein one of the transducers is downstreamfrom the other and wherein there is a pair of oscillators for excitingthe transducers, there being one such oscillator associated with eachtransducer, the improvement comprising: means for selecting one of theoscillators to thereby excite one of the transducers such that only oneof them is excited at any given time in the transmit mode such that anacoustic wave is transmitted across the pipe and is received by theother transducer in the receive mode and such that each of thetransducers is alternatively in the transmit mode and the receive mode;amplitude servo means connected to the transducers such that its inputis connected to the receIve mode transducer and its output is connectedto the transmit mode transducer such that the amplitude of the signalreceived by the receive mode transducer is maintained constant;frequency servo means having its input connected to the receive modetransducer and its output connected to the oscillator associated withthe transmit mode transducer such that the frequency of the saidoscillator is maintained within predetermined limits; and means forobtaining the difference between the frequencies of the two oscillators,which difference frequency is a function of the flow velocity of thefluid in the pipe.
 2. The invention of claim 1 wherein the amplitudeservo means comprises: a single radio frequency amplifier, having aninput and an output, whose input is connected to either of thetransducers whichever is in the receive mode; a pair of differenceamplifiers, each having at least two inputs and an output there beingone such difference amplifier associated with each transducer in thetransmit mode; a pair of driver amplifiers, there being one such driveramplifier associated with each transducer in the transmit mode, eachsuch driver amplifier having at least two inputs and an output; theoutput of the radio frequency amplifier being selectively connected toone of the inputs of the difference amplifier associated with thetransducer in the transmit mode; a reference voltage directly connectedto the other input of both difference amplifiers; the output of eachdifference amplifier being connected to one of the inputs of itsassociated driver amplifier; the other input of each driver amplifierreceiving excitation from its associated oscillator when its associatedtransducer is in the transmit mode.
 3. The invention of claim 2including a peak detector diode connected between the output of theradio frequency amplifier and the inputs to the difference amplifiers.4. The invention of claim 3 wherein the frequency servo means comprises:a control logic counter having an input and a plurality of outputs; theinput of the control logic counter being selectively connected to theoutput of the oscillator in the transmit mode; a threshold detectorhaving an input and an output, its input being connected to the outputof the radio frequency amplifier; a comparator having at least twoinputs and an output; an output of the control logic counter beingconnected to an input of one of the driver amplifiers such that signalis transmitted to the driver amplifier when its associated elements arein the transmit mode; an output of the control logic counter beingconnected to an input of the comparator and the output of the thresholddetector being connected to a second input of the comparator such thatthe signals therefrom are compared to produce a difference therebetweenat the output of the comparator; a second pair of difference amplifiers,there being one such amplifier associated with each transducer, each ofsaid amplifiers having two inputs and an output; the output of thecomparator being selectively connected to one of the inputs of one ofthe pair of the second pair of difference amplifiers whose associatedtransducer is in the transmit mode; a reference voltage connected to theother input of each of the second pair of difference amplifiers so thatthe difference amplifier develops a difference signal between the signalfrom the comparator and the reference voltage; the output of the saiddifference amplifier being connected to its associated oscillator sothat the frequency is adjusted in a predetermined direction dependingupon the sign and amplitude of the difference signal.
 5. The inventionof claim 2 wherein the frequency servo means comprises: a control logiccounter having an input and a plurality of outputs; the input of thecontrol logic counter being selectively connected to the output of theoscillator in the transmit mode; a threshold detector havinG an inputand an output, its input being connected to the output of the radiofrequency amplifier; a comparator having at least two inputs and anoutput; an output of the control logic counter being connected to aninput of one of the driver amplifiers such that signal is transmitted tothe driver amplifier when its associated elements are in the transmitmode; an output of the control logic counter being connected to an inputof the comparator and the output of the threshold detector beingconnected to a second input of the comparator such that the signalstherefrom are compared to produce a difference therebetween at theoutput of the comparator; a second pair of difference amplifiers, therebeing one such amplifier associated with each transducer, each of saidamplifiers having two inputs and an output; the output of the comparatorbeing selectively connected to one of the inputs of one of the pair ofthe second pair of difference amplifier whose associated transducer isin the transmit mode; a reference voltage connected to the other inputof each of the second pair of difference amplifiers so that thedifference amplifier develops a difference signal between the signalfrom the comparator and the reference voltage; the output of the saiddifference amplifier being connected to its associated oscillator sothat the frequency is adjusted in a predetermined direction dependingupon the sign and amplitude of the difference signal.